LONDON (Reuters) - Alexander Zverev claimed one of most significant wins of his fledgling career as he beat Roger Federer to reach the ATP Finals title match but became unwitting pantomime villain in bizarre scenes at the 02 Arena on Saturday.

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The 21-year-old German upstart produced some dazzling tennis and thoroughly deserved his 7-5 7-6(5) win against the six-time champion in a captivating semi-final.

But rather then being able to celebrate becoming the first German since Boris Becker in 1996 to reach the final of a tournament ranked not far below the Grand Slams in importance, he was shamefully booed during his post-match speech.

With Zverev serving at 3-4 in the second-set tiebreak, Federer held sway in a baseline rally when his opponent shouted "hang on" and stopped the point after a ballboy dropped a ball at the far end.

The Swiss looked bemused but after Brazilian umpire Carlos Bernardes checked with the ballboy the point was replayed.

The Federer-loving crowd were in uproar, but Zverev composed himself and slammed down a 137mph ace to level the breaker at 4-4.

Federer then made a hash of an attempted drop volley at 4-5 -- cue more bedlam in the crowd -- and two points later Zverev sealed victory when he belted away a backhand drive volley.

Bizarrely, rather than bask in the glory of beating the 20-time Grand Slam champion on one of his favourite stages, the tousle-haired youngster ended up apologising for a situation not of his making.

"I want to apologise for the situation in the tiebreak," Zverev said on court. "The ball boy dropped ball and it's in the rules that we have to replay the point.

"I apologised to Roger at the net and he said 'it's okay it's in the rules.'"

Boos rung around the arena again, forcing on-court interviewer Annabel Croft to spring to his defence.

"I'm not sure why you are booing, he is telling the truth," she told the crowd. "The ballboy moved across the court and disrupted play. I think you need to be a little bit more respectful, they are the rules."

Federer, who will have to wait a little longer for his 100th career title, still appeared rattled in his post-match news conference when asked if the incident had affected him.

"Of course it did. (The point) got replayed. I got aced," the 37-year-old snapped at a reporter.

He did concede that Zverev had not done anything against the rules.

"I'm not questioning Sascha's sportsmanship in any way," he said. "I think it's a bold move by Sascha to stop the rally because the umpire can just say, 'sorry, buddy, you're in the rally. I don't care. You lost the point. I didn't see it.'

"I don't know what the rule says. I always thought it was an umpire's decision, not a player's decision."

ABSORBING DUEL

The chaotic climax to the contest overshadowed what had been an absorbing duel with Federer's guile and variety up against Zverev's baseline power.

After 11 games without so much as a break point, it was Federer who blinked first.

Zverev curled a superb running forehand pass down the line to get to 0-30 on the Federer serve and was then gifted two points and the set as the second seed's forehand malfunctioned.

Federer was not about to be ushered off without a fight though and a rapier-like backhand pass gave him the early break in the second set, only for Zverev to hit back immediately.

Chants of "Let's Go Roger, Let's Go" reverberated around the arena as the second-set tiebreak began but Zverev kept his cool to prevail.